This invention relates to optical communication components, and more particularly to Mach-Zehnder Modulator pulse generators.
A recent trend in optical communications is to use a Mach-Zehnder Modulator (MZM) as a pulse generator for generating optical pulses. A continuous wave optical signal is inserted into one of two input arms of the MZM. A first coupler splits the continuous wave optical signal into two branch signals, each of which passes along a different branch of the MZM. Driving electrodes operating at a driving frequency alter the phase of each branch signal so that the phase difference between the two branch signals varies. The two branch signals pass through a second coupler, where they are combined to produce an output signal. Because the phases of the branch signals are varied by the drivers, the branch signals will have a variable phase difference upon entering the coupler, and will interfere to a degree that depends on their phase difference.
In this way, pulses are carved from the continuous wave and sent along an output arm as an output signal. The power carved out of the continuous wave is either dissipated, or sent along a second output arm where it is discarded or simply used for monitoring purposes. This is wasted output power which could otherwise be used for improving the power of the output signal.
One characteristic of the pulses is a duty cycle, which is defined as the ratio of the full-width half maximum of a pulse to the period of the pulse. An MZM pulse generator normally operates in one of three states: xe2x80x9cnormally onxe2x80x9d, xe2x80x9cnormally offxe2x80x9d, and xe2x80x9cquadraturexe2x80x9d. In a typical MZM pulse generator, when operating in a xe2x80x9cnormally onxe2x80x9d state the pulses transmitted as the output signal have a duty cycle of 33% when the driving electrodes apply sinusoidal driving signals. When operating in a xe2x80x9cnormally offxe2x80x9d state, the pulses transmitted as the output signal have a duty cycle of 66%. When operating in a xe2x80x9cquadraturexe2x80x9d state, the pulses transmitted as the output signal have a duty cycle of 50%.
There is a growing interest in obtaining a 50% duty cycle, especially for long haul 40 Gb/s transmissions. However, operation of an MZM pulse generator in a quadrature state requires either that the driving frequency of the driving electrodes be the same as the pulse frequency, or that the driving electrodes operate at twice the swing voltage required for a xe2x80x9cnormally onxe2x80x9d state. Both of these solutions require more expensive components and additional power consumption. A 50% duty cycle may also be obtained by altering the split ratios of the couplers or by using non-symmetric electrode lengths in the driving electrodes. However, these methods can introduce chirp to the output signals, to which 40 Gb/s systems are highly sensitive. A 50% duty cycle may also be obtained by using cascaded modulators. However, this requires the introduction of a second active device, increasing the cost and complexity of the pulse generator.
The present invention provides an optical pulse generator which recycles unused power back to the input of the pulse generator. The optical pulse generator receives a continuous wave optical signal and produces a series of pulses. The optical pulse generator includes a Mach-Zehnder Modulator (MZM) having two input arms and two output arms, the first input arm for receiving the continuous wave optical signal and the first output arm for producing the series of pulses. The MZM is configured such that substantially none of a continuous wave optical signal enters the second output arm when the MZM is not being driven by a variable voltage. The optical pulse generator also includes a feedback arm which couples the second output arm to the second input arm. The feedback arm has a length such that a signal travelling in a loop around the MZM and the feedback arm has a propagation time xcfx84 given by   τ  =            (                        2          ⁢          n                +        1            )        ⁢          T      2      
where n is a non-negative integer and T is a pulse period of the pulses. The feedback arm includes a Direct Current phase adjuster.
In one embodiment, the MZM includes a first phase modulation section in a first branch arm, driven by a sinusoidal voltage at a driving frequency equal to half the pulse frequency of the series of pulses. The MZM also includes a second phase modulation section in a second branch arm, driven by a sinusoidal voltage at the driving frequency and 180xc2x0 out of phase with the sinusoidal voltage driving the first phase modulation section.
In another embodiment, the MZM includes two 2xc3x972 couplers, each having a 50xe2x80x9450 splitting ratio and introducing a 0xc2x0 and a 90xc2x0 phase shift, and a phase delay of xcfx80 radians in the first branch arm. The couplers may be Multi-Mode Interference couplers.
The feedback arm may include a splitter leading to a signal monitoring system.
The invention also provides an optical pulse generator which recycles unused power back to the input of the pulse generator. The optical pulse generator receives a continuous wave optical signal and produces a series of pulses. The optical pulse generator includes a Mach-Zehnder Modulator (MZM) having two input arms and two output arms, the first input arm for receiving the continuous wave optical signal and the first output arm for producing the series of pulses. The MZM is configured such that substantially none of a continuous wave optical signal enters the second output arm when the MZM is not being driven by a variable voltage. The optical pulse generator also includes a feedback arm which couples the second output arm to the second input arm. The feedback arm has a length such that a signal travelling in a loop around the MZM and the feedback arm has a propagation time xcfx84 given by   τ  =            (                        2          ⁢          n                +        1            )        ⁢          T      2      
where n is a non-negative integer and T is a pulse period of the pulses. The length of the feedback arm is also such that there is substantially no phase difference between a feedback signal propagating along the feedback arm and the continuous wave optical signal upon entering the MZM
In one embodiment, the MZM includes a first phase modulation section in a first branch arm, driven by a sinusoidal voltage at a driving frequency equal to half the pulse frequency of the series of pulses. The MZM also includes a second phase modulation section in a second branch arm, driven by a sinusoidal voltage at the driving frequency and 180xc2x0 out of phase with the sinusoidal voltage driving the first phase modulation section,
In another embodiment, the MZM includes two 2xc3x972 couplers, each having a 50xe2x80x9450 splitting ratio and introducing a 0xc2x0 and a 90xc2x0 phase shift, and a phase delay of xcfx80 radians in the first branch arm. The couplers may be Multi-Mode Interference couplers.
The feedback arm may include a splitter leading to a signal monitoring system.
The optical pulse generator of the invention allows improved output power of a pulse generator by recycling otherwise discarded power back to the input of the MZM. In addition, a 50% duty cycle can be achieved with the driving frequency of the drivers at only half of the pulse frequency, thereby reducing the cost and power consumption of the pulse generator for a given pulse frequency. If the optical pulse generator is an optical integrated circuit, minimal additional manufacturing steps are required over manufacture of the MZM alone, since the feedback arm is created during the same step as the creation of the waveguides (i.e. the input arms, branch arms, MMIs, and output arms) of the MZM. The optical pulse generator can also be designed to a length comparable to the MZM alone.